A TLS client can not make any assumption about the size of a certificate. Certificates can include images, texts, lists and much more. Assuming that a certificate has a certain size is going to lead to problems in your software.

Certificate with many, many subject Alt Names

This site presents a certificate with a very long list of subjects. The certificate in itself is over 20 kbytes.

This certificate is valid.While most certificates doesn’t have this amount of subjects, there are certificates that have a lot of subjects that a client needs to validate. Check the certificate of https://youtube.com as an example.

Make sure your client goes through the list and validates the site properly.

Fork us on Github

All the tests, including keys and certificates, are available on Github.
https://github.com/edvinanet/tls-o-matic
That's also where you will find all the current tests while waiting for us to write documentation here.

What is TLS?

"The TLS protocol provides communications security
over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery."
From RFC 5446 that defines the current TLS - version 1.2. Wikipedia is also a good help in explaining TLS.